Comstock/Thinkstock(NEW YORK) -- Spondyloarthritis -- the name for a family of inflammatory rheumatic diseases that includes ankylosing spondylitis (AS), a relatively common but incurable form of inflammatory arthritis that strikes young adults -- is just one of many systemic diseases that can first be picked up during an eye exam.

Up to 40 percent of all individuals with AS will experience at least one episode of iritis during the course of their disease.

"Early diagnosis is key to improved outcomes with spondyloarthritis," said Laurie Savage, executive director of the Spondylitis Association of America. "Once diagnosed, an individual can start managing the disease appropriately in partnership with the rheumatologist."

Doctors say dozens, maybe scores of diseases -- from high blood pressure to certain cancers -- can show symptoms in the eye.

Researchers in the United States and in Scotland are even studying how simple eye tests can diagnose illnesses like heart disease.

"There are many systemic diseases we see in the eye," said Dr. Roy Chuck, chair of the department of opthalmology and visual sciences at Albert Einstein College of Medicine and Montefiore Medical Center in New York City.

Diabetes can also be caught early by looking at the eye, as well as advanced hypertension, which shows up as bleeding in new blood vessels and cause retinal detachment.

"You see very skinny blood vessels where the flow is restricted and, in the worst case, bleeding, or new blood vessels responding to not getting enough oxygen," said Chuck. "You see early narrowing to the point where the vessels cross each other."

And it's not just the inner eye that can reveal disease. In xanthelasma, fatty deposits appear on the eyelids. The condition is usually "fairly normal," he said.

But when they appear on only one lid and not the other, it suggests a blood vessel could be blocked off. "It's not a sign of high cholesterol directly, but a blood flow problem," said Chuck.

Even sickle cell anemia, common in African Americans, is visible during an eye exam.

Jaundice often is more prominent in the whites of the eyes, depending on a person's skin tone, and the color can tip off a doctor to liver disease.

But yellowed eyes can also appear "for no reason at all," especially if a person's eyes are blood shot and before the blood clears, the whites turn a kind of "dirty yellow," according to Chuck.

Surprisingly, lesions on the retina can be a sign of Gardner syndrome, a rare genetic disorder characterized by multiple growths or polyps in the colon, extra teeth and bony tumors of the skull.

"When you have specific types of retinal lesions with scarring that are there for more no reason, especially in a younger person, you should ask for a family history of a colon problem," he said.

The eye is quite literally a "real window" to the rest of the body, according to Dr. Noel Bairey Merz, director of the Women's Heart Center at Cedars Sinai Heart Institute in Los Angeles.

"The vitreous fluid is clear and we can look through the opening in the iris and see the blood vessels quite easily," she said. "They taught us in medical school to look with the opthalmoscope as part of the general exam. Sadly, it's not done by most practitioners and they have lost the skill set."

"We have moved away from common things done in a physical exam to higher-tech things that trump the physical exam," she said.

But just this week, Scottish scientists at University of Edinburgh reported that a simple eye test -- taking a high-definition image of the retina -- could help save the lives of thousands of heart attack patients each year by revealing problems with blood vessels that are indicative of cardiac disease.

Bairey Merz has been part of a similar study in the United States -- the NIH-funded Women's Ischemia Syndrome Evaluation -- hoping that by looking at the microvasulature in the eyes, risk can be identified without invasive procedures.

"We look at arteries and veins in the back of the eye as a predictor of heart disease in women and to a lesser extent men," she said. "The idea is that all of these arteries swim in the same swimming pool and are exposed to the same cholesterol level, sugar level, blood pressure, nutrients or lack thereof, exercise and smoking."

Diagnosing illness through the eye is nothing new, according to Dr. Marco Zarbin, chief of ophthalmology at the University of Medicine and Dentistry of New Jersey.

"It happens all the time," he said, from rare conditions to diseases like multiple sclerosis, leukemia, brain tumors.

"If you look at your brain, two-thirds of it is dedicated to some aspect of vision," said Zarbin. "It's a big deal."

Eye doctors emphasize that regular exams are important.

Children get their first eye screenings in public school, but after that, ophthalmologists advise teens get checked once every one or two years, depending on their health.

After 45, when adults start to lose reading vision, yearly visits are recommended.